The present invention relates to a method for manufacturing a scanning optical system which scans a laser beam emitted from a light source by reflecting the beam with reflecting surfaces of a polygon mirror revolving in a fixed direction and converging the reflected beam on a surface to be scanned (hereinafter, referred to as a target surface) by an imaging optical system.
Scanning optical systems are widely used in electrophotographic laser beam printers, digital photocopiers, laser fax machines, laser plotters, and other similar apparatuses for scanning the surface of a photosensitive body (e.g., a photoconductive drum) to scan the target surface with a modulated beam.
Specifically, the scanning optical system dynamically deflects the modulated laser beam, which is ON-OFF modulated according to image information, using the polygon mirror and converges the dynamically deflected laser beam into a beam spot on the target surface by use of an imaging optical system. With the mechanism, the scanning optical system scans the beam spot on the target surface in a main scanning direction at a constant speed. At the same time, the target surface is moved in an auxiliary scanning direction which is perpendicular to the main scanning direction and thereby a two-dimensional image composed of a plurality of dots is formed on the target surface.
Incidentally, in the design of scanning optical systems, how to remove “ghosts” caused by unwanted reflection on surfaces of optical elements of the system is an important matter. Such unwanted reflection can occur, for example, on each surface of each lens forming the aforementioned imaging optical system. Reflected light (ghost light) caused by reflection of part of the laser beam incident upon a lens surface returns in a particular direction that is determined depending on the direction of the laser beam axis and the incident angle of the laser beam relative to the lens surface. If a reflecting surface of the polygon mirror exists in the returning direction of the ghost light, the ghost light is reflected again by the reflecting surface.
If the ghost light from the lens surface is incident upon a reflecting surface of the polygon mirror that is reflecting/deflecting an incident laser beam from the light source, the ghost light is directed in a direction almost opposite to the propagating direction of the incident laser beam, by which the ghost light does not reenter the imaging optical system.
On the other hand, if the ghost light is incident upon a reflecting surface of the polygon mirror that is adjacent to the reflecting surface reflecting/deflecting the incident laser beam from the light source, there is a possibility that the ghost light reflected by the adjacent reflecting surface reenters the imaging optical system and partially reaches the photosensitive body (e.g., photoconductive drum). In such a case, unevenness exposure is caused by the ghost light, which deteriorates the total imaging forming performance of the scanning optical system.
It is possible to remove such ghosts if the reflectivity can be reduced to 0 at all surfaces of the lenses of the imaging optical system. However, reducing the reflectivity closer to 0 requires a larger number of layers for an anti-reflection coating and it drives up the manufacturing costs. Further, in cases of plastic lenses which are recently used for reducing manufacturing costs and for realizing aspherical lens surfaces, adhesion of the anti-reflection coating to the plastic lenses tends to be weaker than that to glass lenses. Therefore, it is undesirable to provide the anti-reflection coating to all lens surfaces of the imaging optical system.